1. Field of the Invention
The present invention related to an image forming apparatus based on electrophotography or electrophotographic recording used, for example, for printers or copiers. In particular, it relates to an image forming apparatus that utilizes a so-called cleaning process simultaneously with developing.
2. Related Background Art
FIG. 6 shows an example of a conventional image forming apparatus. This image forming apparatus is configured by developing images with a one-component developer placed in contact with an image bearing member bearing electrostatic latent images. It does not have any separate cleaning mechanism and it uses the developing device to clean the residual toner on the image bearing member by the so-called cleaning process simultaneous with developing. This conventional image forming apparatus will be described below briefly with reference to FIG. 6.
This image forming apparatus comprises a photosensitive drum 101 which is an image bearing member for holding electrostatic latent images, exposing means 102 for recording electrostatic latent images on the photosensitive drum 101, a developing device 103 for developing the electrostatic latent images on the photosensitive drum 101 as well as for collecting any residual toner that was not transferred to the transferring material and cleaning its history, a transfer roller 104 which is transfer means for transferring the toner images developed on the photosensitive drum 101 to the transferring material, a heat roller 105 which is fixing means for fixing the toner images transferred to the transferring material, etc.
On this image forming apparatus, images are formed as follows. First, image information is recorded as an electrostatic latent image on the photosensitive drum 101 by the exposing means 102. The electrostatic latent image on the photosensitive drum 101 is visualized as a toner image, being developed by the developer supplied by the developing device 103 placed in abutment with the photosensitive drum 101, under the action of the developing electric field generated by a power supply not shown. The toner image is transferred to the transferring material under the action of the transfer electric field generated between the photosensitive drum 101 and transfer roller 104 and is fixed on the transferring material under the heat and pressure applied by the heat roller 105. The remainder of the toner image (residual toner) that was not transferred to the transferring material and left on the photosensitive drum 101 is brought into abutment with the developing device 103 again and collected to the developing device 103 for reuse under the action of the developing electric field described above.
At the time of developing electrostatic latent images in the image formation process described above, elastic bodies such as rubber rollers and sponge rollers are used for the developer carrying member 111 provided in the developing device 103 because the developer is brought into contact with the photosensitive drum 101.
FIG. 7 shows an enlarged view of the developing device 103.
The developing device 103 mainly comprises a developing roller 111 which is a developer carrying member for carrying a one-component developer on its surface and which rotates in the direction of the arrow in the figure, a regulating blade 112 which is a developer regulating member for regulating the developer, a feed roller 113 which serves both as developer supply means for supplying the developer to the developing roller 111 rotating in the direction of the arrow in the figure and as developer recovery means for collecting the developer not used for developing, an agitating paddle 114 for agitating and carrying the developer, a spout-proofing sheet 115 for preventing the developer from scattering from the lower part of the developing roller 111, and a developer container 116 for housing the developer.
Now, the operation of the developer 103 will be described.
The developer housed in the developer container 116 is carried to the area R near the developing roller 111 and feed roller 113 by the agitating paddle 114. The developer carried there is homogenized by being made denser once in the area R with the rotation of the feed roller 113. Then it si given a charge by the frictional electrification resulting from rubbing with the developing roller 111 and feed 113 counterrotating in abutment with each other.
The developer given a charge in this way is supplied to the developing roller 111 by the reflection force received from the developing roller 111 through the acquired charge. Thin layers of the developer are applied to the developing roller 111 under the regulation of the regulating blade 112 pressed against the developing roller 111. In this way, the charge for developing is given by the frictional electrification with respect to the surfaces of the developing roller 111 and blade 112.
Through these actions, thin layers of charged developer are applied to the developing roller 111 for use in developing. The developer applied to the developing roller 111 but not used for developing is scraped off the developing roller 111 by the feed roller 113. Part of it is supplied again to the developing roller 111 by the feed roller 113 together with fresh toner and the rest is returned to the developer container 116.
On the other hand, the photosensitive drum 101 is placed in abutment with the developing roller 111 and thus the developer applied to the developing roller 111 is used for developing while being in contact with the photosensitive drum 101. A power supply not shown is connected between the photosensitive drum 101 and developing roller 111 so as to form a developing electric field. The developer applied to the developing roller 111 is transferred to the electrostatic latent image borne on the photosensitive drum 101 under the action of this electric field to develop the image.
At the same time, in the abutment area between the photosensitive drum 101 and developing roller 111, the residual toner produced as a result of the image formation is collected. The residual toner in the abutment area is transferred to the developing roller 111 by the developing electric field acting strongly between the very closely located photosensitive drum 101 and developing roller 111. There the residual toner is collected.
The image formation process proceeds through the sequence of actions described above.
However, the image forming apparatus described above, where electrostatic latent images are developed with the one-component developer carried on the developing roller 111 kept in contact with the photosensitive drum 101, is liable to produce image fog because toner that is contained in the developer carried by the developing roller 111 and that is charged insufficiently (uncharged toner) or charged oppositely (reverse toner) to the desired polarity will cling to the background and part of it will be transferred to the transferring material.
On the other hand, it is possible to reduce image fog by making the layers of developer applied to the developing roller 111 very thin and allowing sufficient frictional electrification, the history of previous image formation or so-called ghost may appear if the cleaning process simultaneous with developing is used. This will be described in detail below.
According to studies conducted by the inventors, if the layers of the developer carried on the developing roller 111 are made very thin, sufficient frictional electrification can decrease uncharged toner or reverse toner and thus reduce image fog. However, it was found that since very thin layers of developer are applied to the developing roller 111, the history generated by the residual toner collected during the cleaning process simultaneously with developing would appear as local variations in the amount of developer carried on the developing roller 111, resulting in a ghost during the developing process going on simultaneously.
Also, this type of ghost tends to become prominent as the image fog decreases. This is because when images are fogged badly, a great amount of fog-causing toner clings to the background, burying residual toner in the noise on the background and thus preventing the history of the residual toner from standing out alone.
Furthermore, the toner collected during the cleaning process simultaneous with developing has undergone functional deterioration under the load involved in the sequence of image formation processes and part of it is accumulated in the developing device without reuse, causing functional deterioration of the members composing the developing device. Therefore, it is desirable that the absolute amount of the toner collected is small, for example, in order to speed up image formation or prolong the life of the apparatus. In this respect, it is indispensable to reduce uncharged toner or reverse toner that will cause image fog.
In other words, there is the problem that in the performing cleaning process, simultaneously with developing where thin layers of a one-component developer are applied the developing roller 111 and the developer and the developing roller 111 are placed in contact with the photosensitive drum 101, it is not easy to reduce image fog and ghosts. This in turn presents the problem that it is not easy to speed up image formation or prolong the life of the apparatus while maintaining good image formation.
An object of the present invention is to provide an image forming apparatus that will allow good image formation.
Another object of the present invention is to simultaneously reduce image fog and ghosts and provide an image forming apparatus capable of maintaining good image formation for an extended period of time.
Still another object of the present invention is to provide an image forming apparatus, comprising an image bearing member for bearing an electrostatic latent image; and developing means for developing the electrostatic latent image with a developer and collecting the developer remaining on the image bearing member, including a developer carrying member for carrying the developer wherein the developer carried by the developer carrying member comes into contact with the image bearing member as the developer carrying member moves, and a regulating member for regulating a layer thickness of the developer carried by the developer carrying member wherein the regulating member regulates the layer thickness of the developer carried by the developer carrying member so that there will be three to eight layers.
Other objects and features of the present invention will be more apparent by reference to the following detailed description taken in connection with the accompanying drawings.